The 2017 return of 96P/Machholz, what did we see?

On October 27th, 2017 comet 96P/Machholz reached its 6th Perihelion passage since it’s discovery in 1986. At the time Don Machholz discovered this comet it had already passed perihelion. 96P/Machholz is a rather unusual comet, in the case of its composition (reltaively low in C2, C3 and Cyanogen) and unusual orbit for a short periodic comet (very inclinated and eccentric).

In this (unusually long) post I will go into some depth regarding the inbound journey and perihelion passage of 96P/Machholz as seen by the Solar and Heliospheric Observatory (SOHO), Solar TErrestrial RElations Observatory Ahead (STEREO-A) and by some ground-based observations.

Nota bene: This is an updated version of a Blog post published on this website during the beginning of November.

Ground-based Observations

Before perihelion, the comet was only visible from the Southern-Hempisphere, quickly moving North and rapidly brightening. The comet wasn’t observable for very long, as it stayed extremely faint until about a month before perihelion. Soon, by mid-October, it entered daylight as seen from Earth.

Comet 96P/Machholz imaged by on October 2nd with C11 at f/6.3 + QHY 163m ccd. The comet was estimated to be about mag +15 in this image. (c) Michael Mattiazzo.

Michael Mattiazzo’s (Australia) magnitude measurments from October 8th to October 16th are listed below. Also, on October 2nd he estimated the comet to be about +15 mag. Rob Kauffman (Australia) estimated it to be around +13 mag on October 10th, well in accordance well Michael’s measurments.

Photometric and astronomic measurments of 96P/Machholz by Michael Mattiazzo. Notice the rapid brightening of this object as it rapidly moves towards perihelion.

SOHO/SWAN Observations

Days later the comet was already close to +11 mag, and was hence detectable in SOHO’s SWAN images. In these images it was possible to follow up on the brightening, as the object had passed within the Daylight zone as seen from Earth. By late October the comet passed behind the “Sun-Occulted zone”, meant to block out the intense Ultraviolet sunlight from saturating the SWAN images.

STEREO HI1-A Observations

At about the same time the comet was passing behind the occulter in SWAN images, on October 23rd, it entered the FOV of the STEREO/SECCHI HI1-A imager. In these images the comet was bright. It was also showing a long (several degrees, and growing) prominent tail. Consequently, by the time the comet had left the FOV on October 24th, the tail was still visible after many hours. Furthermore, one could clearly see the impact of the Solar wind on the comet’s tail, causing it to appear discontinous and curvy.

Comet 96P/Machholz entering STEREO’s HI1-A FOV (October 23rd) and the comet on October 25th when it had already left the FOV. Notice how the tail still persists almost a day after the comet left the FOV! Furthermore, notice the tail’s discontinous and curvy nature, caused by its interaction with the Solar Wind. Image credit: NASA STEREO/SECCHI HI1-A.

SOHO/LASCO observations

On october 25th, only a couple of days before perihelion, the comet entered the SOHO LASCO C3 images. Based on these images the comet appeared close to mag +2. It kept at about this brightness during its passage in SOHO/LASCO. The comet left the SOHO/LASCO C3 FOV on October 29th. In these images it was possible to distinguish two tails.

Between October 27th and 29th, amateurs reported three faint fragments leading the comet in SOHO/LASCO C3 images. Two of them correspond to 96P/Machholz-B and 96P/Machholz-C, discovered during the comet’s passage in SOHO/LASCO in 2012. The third fragment is a new discovery. The comet’s were solely observed by SOHO, not even STEREO/SECCHI images could detect them.

SOHO/LASCO C3 (long-exposure) extract showing 96P/machholz and its three faint leading fragments. Two of them correspond to 96P/Machholz-B and C, discovered in 2012. The third one is a previously unknown fragment. Image credit: ESA/NASA/NRL Karl Battams.

Fragmentation events are nothing new to this comet. In fact, it’s likely that the precursors of the Marsden and Kracht-I comet groups were chunks that broke off from 96P/Machholz somewhere around 1000 years ago.

An example of a bright Marsden group comet as seen in a SOHO/LASCO C2 image extract. This comet is possibly a piece of a much a larger chunk that broke off from 96P/Machholz a millenium ago. Image credit: ESA/NASA SOHO/LASCO C2.

STEREO COR-A Observations

On October 26th comet Machholz entered the STEREO/SECCHI COR2 FOV. In these images the comet was a prominent object. At its best it appeared to be around mag +2 and showed a long and prominent tail. The comet left the FOV on October 28th.

The comet also rapidly transited the COR1-A FOV on October 27th, near perihelion. Unfortunately the comet was hardly detectable in these images, as the it was practically on the other side of the Sun as seen from STEREO-A.

STEREO COR1-A image extract showing 96P/Machholz. In these images the comet was hardly detectable, this image is perhaps one of the better COR1-A images of it. Image credit: NASA STEREO/SECCHI COR1-A.

Outbound

According to Alan Hale (USA), the comet is expected to stay unobservable from the ground until about February 2018, when the comet will start entering the night sky again. By then unfortunately the comet will have become very faint. Luckily, until then, the SOHO/SWAN images has a good view of the comet. The comet reappeared in SWAN during the beginning of November, appearing bright but fading, while heading Eastward.

SOHO/SWAN image extract of 96P/Machholz only days after it reappeared from behind the occulter. Despite the low resolution of the images, a faint tail is still apparent. Image credit: ESA/NASA/LATMOS SOHO/SWAN.

Acknowledgements

I wish to thank Charles Bell, Alan Hale, Michael Mattiazzo and Ian Musgrave for the information they provided to help write (and correct) this post!